Test and Measurement
peaks and limit the control signal to 3 volts to 5.5 volts, while RC filters ensure a residual ripple of less than ±0.5 volts.
Reducing leakage currents The Cout of PhotoMOS relays serves as a bypass for alternating currents and pulse sequences of higher frequency when the relay is de-energized. To significantly reduce such leakage currents and maximize isolation at high frequencies, Panasonic recommends using three separate PhotoMOS relays in the form of a T-circuit (Figure 5, left). In the main signal path, the two 1 Form A PhotoMOS relays, S1 and S2, are low-Ron types, while a low-Cout type forms the 1 Form A short-circuit switch, S3.
Figure 3: Each signal path of this automated semiconductor test system requires a specific PhotoMOS relay type. (Image source: Panasonic) continued from page 22
contact (SPST-NO)) or form B (normally closed contact, SPST-NC), and as multiples. Designers can build form C switches such as single pole, double throw (SPDT); single-pole changeover switches; and double pole, double throw (DPDT) switches by combining form A and form B devices.
Figure 4: SSRs and photocouplers cause distortions in the output signal due to threshold and ignition voltages; PhotoMOS relays switch AC and DC signals without distortion. (Image source: Panasonic, modified by author)
For example, the AQS225R2S is a quadruple PhotoMOS relay (4SPST-NO) in a SOP16 housing that can handle a maximum of 70 mA at switching voltages up to 80 volts. Also, the AQW214SX is a dual PhotoMOS relay (2SPST- NO) in a SOP8 housing that can handle load currents up to 80 mA at switching voltages up to 400 volts.
T-circuit ON state (Figure 5, centre): In the case of S1 and S2 switched on, their Ron minimally attenuates the signal level, while the low Cout from the switched-off S3 relay slightly attenuates high frequencies (low pass). T-circuit OFF state (Figure 5, right): If S1 and S2 are de-energized, their Cout represents a bypass for high frequencies (high pass), but the switched-on S3 relay short circuits the signals capacitively passed through S1 (suction circuit).
The ON/OFF timing of the T-circuit must be implemented as a break before make (BBM) switch. Accordingly, S1 and S2 should be deactivated before S3 is turned on. With relays, BBM means that the contacts switch over separately, whereas make before break (MBB) means they switch over in a bridging manner.
Switching PhotoMOS relays faster The internal photosensor of the PhotoMOS relay works as a solar cell and supplies the gate charging current. As such, a brighter light pulse from the LED increases the switching speed. For example, the bootstrap element R1/R2/C1 in Figure 6 generates a higher current pulse.
Figure 5: When S1 and S2 are de-energized, the switched-on relay S3 acts as a short circuit for all leakage currents (T-circuit OFF state, right). (Image source: Panasonic, modified by author)
Figure 6: The bootstrap element R1/R2/C1 increases the switch-on speed of the PhotoMOS relay. (Image source: Panasonic)
Figure 4 shows the internal structure of an SSR, PhotoMOS, and a photocoupler, along with their typical signal distortions. PhotoMOS relays do not cause signal clipping or similar distortions on ohmic loads.
To attenuate the feedback effect of inductive and capacitive switching loads, thus protecting the PhotoMOS output stage, designers must add clamp and freewheeling diodes, RC and LC filters, or varistors on the output side. In the CC series, clamp diodes protect the input oscillator from overvoltage
24 October 2024 Components in Electronics
C1 acts as a short circuit for R2 at the moment of switch-on, so the low resistance of R1 allows a high current to flow. If C1 is charged and has a high resistance, R2 is added, reducing the flow to the holding current, as with magnetic relays. The AQV204 PhotoMOS relay thus reduces its switch-on time from 180 µs to 30 µs.
Conclusion
By using small, wear-free PhotoMOS relays, designers can improve ATE applications' signal density and measurement speed while reducing maintenance needs. Additionally, following recommended design techniques can help minimize leakage currents and switching times.
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